feat(lsp): support CompletionItem.labelDetails #38403

Problem: CompletionItem.labelDetails is ignored, losing
function signatures and module info in the completion menu.

Solution: Append labelDetails.detail to abbr and use
labelDetails.description for menu with fallback to item.detail.

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#completionItemLabelDetails

runtime/lua/vim/lsp/protocol.lua

@@ -488,6 +488,7 @@ function protocol.make_client_capabilities()
           tagSupport = {
             valueSet = get_value_set(constants.CompletionTag),
           },
+          labelDetailsSupport = true,
         },
         completionItemKind = {
           valueSet = get_value_set(constants.CompletionItemKind),